Implement T.4 MH (Modified Huffman) encoder

- encoder/mh.rs: Full T.4 Group 3 MH encoding implementation
- Uses ITU-T T.4 standard Huffman tables for white/black runs
- Adds EOL codes and RTC (6 EOLs) at document end
- Class2Send now encodes pixels to MH before transmission
- CLI convert command shows MH encoded size

Test result: 467KB raw pixels → 14.6KB MH encoded (~32:1 compression)

src/fax/class2/send.rs

@@ -1,5 +1,6 @@
 use crate::error::{FaxError, Result};
 use crate::fax::class2::commands::{Class2Commands, DccParams, BR_V17_14400};
+use crate::fax::encoder::MhEncoder;
 use crate::modem::driver::ModemDriver;
 use crate::document::convert::Page;
 
@@ -128,20 +129,13 @@ impl<'a> Class2Send<'a> {
             return Err(FaxError::modem("FDT command failed"));
         }
 
-        // Send T.4 MH encoded data
-        // For Class 2, we need to:
-        // 1. Send T.4 page data (with proper MH encoding)
-        // 2. End with DLE ETX (0x10 0x03)
-        // 3. Handle any DLE escapes in the data
+        // Encode pixel data to T.4 MH format
+        tracing::info!("Encoding {}x{} pixels to T.4 MH...", page.width_pels, page.rows);
+        let mh_data = MhEncoder::encode_page(page)?;
+        tracing::info!("Encoded to {} bytes", mh_data.len());
 
-        // TODO: The pixel data needs to be T.4 MH encoded first
-        // For now, send raw pixel data (this won't work properly!)
-        // We need to implement MH encoding
-
-        tracing::warn!("Sending raw pixel data (MH encoding not yet implemented)");
-        
         // Escape any DLE (0x10) bytes in the data by doubling them
-        let escaped_data = self.escape_dle(&page.pixels);
+        let escaped_data = self.escape_dle(&mh_data);
         
         self.driver.write_raw(&escaped_data)?;
         self.driver.write_raw(&DLE_ETX)?;